Is it a bad idea to put a one year old male guinea pig with another young 3-5 months old male guinea pig together in the same cage?

It is not a smart idea at all to put them together

BECAUSE THEY MIGHT FIGHT!

well if u want your pet to have a friend it is a good idea ...yes sometime male guinea pigs will fight but not very often....they r more likely to fight if they are the same age ....so by adding a new younger guinea pig with an older one is the best way and best chance they will make the best of friends ...just always make sure...there is always plenty of food and fresh water, if u need to add 2 bowls of food to the cage or hutch then do so ....male guinea pigs also seem to need more space then females so a nice big hutch or cage xxx